# Breaker sizing and short-circuits

Breaker sizing is another basic skill required for electrical engineers in the nuclear power industry. Here’s the short of it . . . .

In accordance with the NEC requirements, circuit breakers are typically sized to be 1.25 times the full-load current of continuous loads, similar to the ampacity ratings (100% of any noncontinuous load current must be added to this). In the case of the circuit breaker rating, most molded-case circuit breakers aren’t intended to be loaded greater than 80% of their rating (unless specifically listed for use at 100% of their rating, as explained in Article 210.20(A) of NEC 2014). Thus, a molded-case circuit breaker rated at 20 amps can handle a maximum continuous load of 0.8 x 20 = 16 amps (unless there are certain exceptions).

The basic NEC articles that address these requirements are:

•  Article 210.20 for branch circuits
• Article 215.3 for feeder circuits
• Article 430 for motors and motor circuits

The inverse of 0.8 is 1.25. The minimum required breaker rating is determined by multiplying the full load by 1.25. If the load is 13.7 amps, then the minimum breaker size needs to be 13.7 x 1.25 = 17.125 amps. This is not a standard breaker rating, so per the NEC you would select the next standard size up, which in this case is 20 amps. This procedure is explained in “Overcurrent Devices Rated 800 Amperes or Less,” of Article 240.4(B) in NEC 2014.

For a circuit that has a continuous full load of 13.7 amps and a noncontinuous load of 4 amps, the minimum breaker size must be (13.7 x 1.25) + 4 = 21.125 amps. Per Article 240.6, the next standard size up is 25 amps, so that’s what you select.